In the USA, Squadron.com and rollmodels.com, as well as avusk.net will have a lot of detail sets. As for applications, some sets are dedicated, e.g. Hasegawa P-47 Bubbletop in 1/48, others, such as the Eduard seatbelt sets, or radio sets are more generic in character. All the above retailers make it clear which sets are only for certain kits. Eduard, Verlinden, Part, Aber CMK, Airwaves, FM Details are all quality manufacturers, and the Squadron True Details line is very useful, and bargain priced too.